1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same, and more particularly, to a thin film transistor and a method for fabricating the same.
2. Description of Related Art
In a device, a switch is formed for controlling the operation of the device. As to an active display device, a thin film transistor (TFT) is generally made as a switch. Based on the material of the channel of thin film transistors, there are two types of TFT: amorphous silicon (a-Si) thin film transistor and polysilicon thin film transistor. A polysilicon thin film transistor has lower consuming power and higher electromigration than an amorphous silicon thin film transistor. The polysilicon thin film transistor receives increasing attention in the market after laser technology is employed for fabricating the polysilicon thin film transistor under 600 centigrade degrees.
FIG. 1 is a schematic cross-sectional drawing of a conventional thin film transistor. Referring to FIG. 1, after a buffer layer 102 is formed on a substrate 100, a polysilicon layer 104 and a gate dielectric layer 106 are sequentially formed over the buffer layer 102. A gate layer 108 is formed on the gate dielectric layer 106. Thereafter, a source region 1110a and a drain region 110b are formed using a doping method with a doping mask. A channel region 112 is formed under the gate layer 108. Leakage current and hot carrier effect can be avoided by forming lightly-doped-drain (LDD) regions 114a and 114b between the source region 110a and the channel region 112 and between the drain region 110b and the channel region 112. A source conductive layer 118a and a drain conductive layer 118b passing through the gate dielectric layer 106 and a dielectric layer 116 are electrically connected to the source region 110a and the drain region 110b, respectively.
In the above process, leakage current and hot carrier effect can be mitigated by the lightly-doped-drain (LDD) regions that are formed using an extra ion implanting process with an extra doping mask. In other words, the source region/drain region and the lightly-doped-drain (LDD) regions are formed using two steps of the ion implanting process with two different doping masks formed by different masking processes. Therefore, the process for fabricating the thin film transistor with the lightly-doped-drain (LDD) regions is complicated and time-consuming. The mismatch of the positioning of the different doping masks is resulted during the fabrication of the thin film transistor.